If you looked at an undisturbed sedimentary column, the remains of human beings would be found only in the very topmost layers. The farther down you dig, the farther back in time you are going. And no one has ever found any remnant of a human being down in the Jurassic or the Cambrian or any of the geological time periods other than the most recent-the last few million years. And likewise there are many organisms that were absolutely dominant and abundant worldwide for enormous periods of time that became extinct and were never seen again in the higher sedimentary columns. Trilobites are an example. They hunted in herds on the ocean bottoms. They were enormously abundant, and there have not been any of them on the Earth since the Permian. In fact, by far most of the species of life that have ever existed are now extinct. Extinction is the rule. Survival is the exception.
When you look at the fossil record, it is clear that some organisms have powerful anatomical similarities with others. Others are more distinct. There is a kind of taxonomic evolutionary tree that has been painstakingly developed over a century or more. But in recent times it is possible to look for chemical fossils-to examine the biochemistry of organisms that are alive today-and we are even just beginning to know something about the biochemistry of organisms that are extinct, because some of their organic matter can nevertheless be recovered. And here there is a remarkable correlation between what the anatomists say and the molecular biologists say. So the bone structure of chimpanzees and humans is startlingly similar. And then you look at their hemoglobin molecules, and they are startlingly similar. There's only one amino acid difference out of hundreds between the hemoglobins of chimps and humans.
In fact when you look more generally at life on Earth, you find that it is all the same kind of life. There are not many different kinds; there's only one kind. It uses about fifty fundamental biological building blocks, organic molecules. (By the way, when I use the word "organic," there is no necessary implication of biological origin. All I mean when I say organic is a molecule based on carbon that's more complicated than CO and C02.)
Now, it turns out that with trivial exceptions all organisms on Earth use a particular kind of molecule called a protein as a catalyst, an enzyme, to control the rate and direction of the chemistry of life. All organisms on Earth use a kind of molecule called a nucleic acid to encode the hereditary information and to reproduce it in the next generation. All organisms on Earth use the identical code book for translating nucleic acid language into protein language. And while there are clearly some differences between, say, me and a slime mold, fundamentally we are tremendously closely related. The lesson is, don't judge a book by its cover. At the molecular level, we are all virtually identical.
This then raises interesting questions about whether we have any idea of the possible range of life, of what could be elsewhere. We are trapped in a single example and have not the imagination to guess even one other way in which life might exist when there might be thousands or millions. Certainly no one deduced from fundamental theoretical chemistry the existence and function of nucleic acids when they were all around us and, in fact, when we ourselves were made of them.
Now, how did it come about that these few particular molecules, out of the enormous range of possible organic molecules, determine all life on Earth? There are two main possibilities and a range of intermediate cases. One possibility is that these molecules were somehow made preferentially in great abundance in the early history of the Earth, and so life just used what was lying around.
The other possibility is that these molecules have some special properties that are not only germane but essential for life, and so they were gradually developed by living systems or preferentially removed from a dilute to a concentrated solution by them. And, as I said, there is a range of intermediate possibilities.
It would be wrong to say that the origin of proteins and nucleic acids is identical with the origin of life. And yet nucleic acids are known in the laboratory to replicate themselves and even to replicate changes in themselves from plausible building blocks in the medium. It is true that an enzyme is needed for this reaction in the laboratory, but this enzyme determines the rate and not the direction of the chemical reaction, so it merely shows us what would happen were we willing to wait long enough. And there was surely plenty of time for the origin of life, which I will come back to as well.
It is certainly conceivable that what we have today is quite different from what was present at the time of the origin of life. We have today a very sophisticated kind of life, evolved by natural selection, that was based upon something much simpler, much earlier. It has been proposed that "much simpler" might in fact be mainly inorganic or it may have been organic; there is no way to be sure. But one thing is undoubtedly of interest for the origin of life-some would say essential-and that is to understand where the molecular building blocks that are present in all living things today came from.
So we now come to the issue of organic molecules. They are found on the Earth, of course, but since the Earth is littered with life, we do not have a clean experiment. We don't know, or at least it's not immediately obvious, which organic molecules we see on the Earth are here because of life and which would be here even if there had not been life. And virtually all the organic molecules that we see in our everyday lives are of biological origin. If you want to know something about organic chemistry on the Earth prior to the origin of life, it is a good idea to look elsewhere.
The idea of extraterrestrial organic matter is important not just for this reason but also because it tells us something relevant at least about the likelihood of extraterrestrial life. If it turns out that there is no sign of organic molecules elsewhere, or they're extremely rare, that might lead you to conclude that life elsewhere was extremely rare. If you found the universe burgeoning and overflowing with organic matter, then at least that prerequisite for extraterrestrial life would be satisfied. So it's an important issue. It's an issue where remarkable progress has been made since the early 1950s, and it speaks to us, I believe, if not centrally at least tangentially, about our origins.
The astronomer Sir William Huggins frightened the world in 1910. He was minding his own business, doing astronomy, but as a result of his astronomy (the work I'm talking about was done in the last third of the nineteenth century) there were national panics in Japan, in Russia, in much of the southern and midwestern United States. A hundred thousand people in their pajamas emerged onto the roofs of Constantinople. The pope issued a statement condemning the hoarding of cylinders of oxygen in Rome. And there were people all over the world who committed suicide. All because of Sir William Huggins's work. Very few scientists can make similar claims. At least until the invention of nuclear weapons. What exactly did he do? Well, Huggins was one of the first astronomical spectro-scopists.
fig. 20
This is the coma of a comet-the cloud of gas and dust that surrounds the icy comet nucleus when it enters the inner solar system. Huggins used a spectroscope to spread out the light from a comet into its constituent frequencies. Some frequencies of light are preferentially present, from which it is possible to deduce something of the chemistry of the material in the comet. This is an application of stellar spectroscopy that had been going very successfully in the decade or two before Huggins turned his attention to the comets. (Huggins also made major contributions to understanding the chemistry of the stars.)